This invention generally relates to toner compositions and a dispersion dyeing method of producing toners for developing latent electrostatic images in electrophotography, electrostatic recording and electrostatic printing. More specifically, this invention relates in preferred embodiments to micro-serrated dyed color toner compositions and a co-solvent-mediated dispersion dyeing method of suitably sized resin particles to form toner particles for high-resolution electrophotography, electrostatic recording and electrostatic printing.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process (U.S. Pat. No. 2,297,691) involves placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic toner material. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image subsequently may be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Toners and developer compositions including colored particles are well known. Electrostatic images formed on an electrophotographic photoconductor and an electrostatic recording medium are generally developed by using (i) a mono-component toner composition comprising a binder resin, a coloring agent such as a dye or pigment and a charge control agent thereto when necessary or (ii) a two-component type developer composition comprising a toner composition blended with solid carrier particles. Some U.S. patents in this regard are U.S. Pat. Nos. 5,352,521, 4,778,742, 5,470,687, 5,500,321, 5,102,761, 4,645,727, 5,437,953, 5,296,325 and 5,200,290. The traditional compositions normally contain toner particles consisting of resin and colorants, wax or a polyolefin, charge control agents, flow agents and other additives. A typical toner formulation generally contains about 90-95 weight percent resin, about 2-10 weight percent colorant, from about 0 to about 6 weight percent wax, from about 0 to about 3 weight percent charge control agent, about 0.25-1 weight percent flow agent and from about 0 to about 1 weight percent other additives. Major resins are styrene-acrylic copolymers, styrene-butadiene copolymers and polyesters. The colorants usually are selected from cyan dyes or pigments, magenta dyes or pigments, yellow dyes or pigments, and mixtures thereof.
One of the main advantages of selecting organic dyes instead of pigments for color toner compositions resides in the provisions of increased color fidelity as the dyes can be molecularly dispersed in the toner resins. To obtain a homogeneous dispersion, it is generally necessary to build into these molecules certain substituents for enhancing their compatibility with the toner resin. Unless the dye molecules are substantially fully compatible with the toner resins, they have a tendency to aggregate with time, especially when subjected to heat, pressure and humidity thereby resulting in a loss of color fidelity. Additionally, the low molecular weight of the dye molecules causes a high lability or mobility of the dye molecules in the toner resin resulting in undesirable bleeding of the dyes.
An attempt for improvement is to incorporate a dye into preformed resin particles by dispersing the particles in a dye solution and diffusing the dye into the central portion of each resin particle. For example, U.S. Pat. No. 5,565,298 discloses a method of producing toner particles comprising of a copolymer of styrene and n-butyl methacrylate formed by a suspension polymerization method and dyed by dispersing in a bath comprising of a dye and methanol as solvent. However, the method has several deficiencies that make it unsuitable for producing high-resolution toner particles. The dyeing has to be carried out below the glass transition temperature of the resin and it therefore takes a long dyeing time. Particles also tend to coagulate in the course of dyeing resulting in a large average particle size and a broad size distribution. Incorporating a sufficient amount of dyes for vivid color image is difficult due to a limited solubility of dyes in polymer resins. Dyes tend to migrate out of the particle during storage and evaporate during the fixing stage of electrophotography process, severely interfering with operation of electrophotography equipment.
A dispersion dyeing process for particulate resin is disclosed in a co-pending patent application, Ser. No. 09/457,543. The invention provides a method of producing high-resolution color toner by dispersing resin particles and a dye in a bath and effecting the dye molecules to be absorbed in the central portion of each resin particle while substantially maintaining the size and size distribution of the resin particles. The polymer resin contains functional groups in its molecular structure for interacting with a functionalized dye in order to effect a deep dyeing as welt as to bind to enhance dye fastness. The bath comprises an organic solvent which is immiscible with the polymer resin and a non-ionic surfactant. The surfactant not only prevents the resin particles from coalescing but provides a solubility of the dye so that dyeing can be effected. There are, however, several disadvantages associated with the invention. Dyes usable for the invention are limited to the ones that have solubility in the surfactants. Furthermore, the functionalized dyes generally are not soluble in the organic solvents used in the invention and have a very limited solubility in the surfactants. The dyeing process therefore is slow and has to be carried out at an elevated temperature which is typically about 40xc2x0 C. above the glass transition temperature of the resin. As a result, the toner particles produced by the process tend to have a spherical shape and a smooth surface texture. Consequently, the toner composition of the invention tends to have a slow triboelectric charging characteristic and is not advantageous for use in a mono-component electrophotographic development system.
There is continuing interest in the development of new and improved methods of producing toners for application in high-resolution color electrophotography.
Accordingly, an object of the present invention is to provide a method of producing high-resolution color toner which has a superior combination of properties for electrophotographic imaging systems by dispersing resin particles and a dye in a bath comprising an organic solvent, a surfactant and a dye-mediating co-solvent and expeditiously effecting the dye molecules to be absorbed in the central portion of each resin particle while substantially maintaining the size and size distribution of the resin particles as well as desirable surface properties.
Another object of the present invention is to provide a method of producing high-resolution toner of which toner particles are substantially spherical and have micro-serrated surface texture.
Yet another object of the present invention is to provide a high-resolution color toner composition of which particles are spherical in shape with a diameter in the range of about 1 to 10 microns, have a narrow size distribution and have a micro-serrated surface texture.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.
There is provided in accordance with the present invention a color toner composition including dyed resin particles having a volume average diameter in the range of from about 2 to about 10 microns with a size distribution span value of less than 1.0 and a micro-serrated surface texture. Generally, the particles exhibit a surface roughness index value of greater than about 1.2. Surface roughness index values of greater than about 1.5 or about 2.0 or more may readily be achieved if so desired.
There is provided in accordance with another aspect of the present invention a process of preparing a toner for developing latent electrostatic images comprising: dispersing a particulate polymer resin with functional sites suitable for interacting with a functionalized dye in a liquid organic medium; the organic medium comprising an organic solvent or dispersion medium, a surfactant, and a dye-mediating co-solvent; the polymer being substantially insoluble in the organic dispersion medium; providing a functionalized dye to the organic medium wherein the functionalized dye has functional sites adapted for interacting with the functional sites on the particulate polymer resin; maintaining the organic medium containing the particulate resin for a time sufficient to dye the resin and separating the organic medium from the particulate polymer resin. The functionalized dye is thus applied to the resin particles and the particle size of the particulate polymer resin is substantially unchanged during the dyeing process recited above.
Any suitable polymer resin for toner application may be used. The polymer resin may have functional sites suitable for interacting with a functionalized dye selected from the group consisting of: hydroxyl moieties; alkoxyl moieties; sulfonic or derivatized sulfonic moieties; sulfonic or derivatized sulfonic moieties; carboxyl or derivatized carboxyl moieties; phosphonic or derivatized phosphonic moieties; phosphinic or derivatized phosphinic moieties; thiol moieties, amine moieties; alkyl amine moieties; quaternized amine moieties; and mixtures thereof.
In typical embodiments the particulate polymer resin has a volume average particle size of from about 2 to about 10 microns. The particulate polymer resin preferably has a volume average particle size of from about 3 to about 8 microns. It is generally preferred that the particulate polymer resin has a narrow size distribution with the 80% span (the xe2x80x9cspanxe2x80x9d) less than 1.0, the 80% span being defined as the ratio of the middle size range in which 80% of the particles exist to the volume average particle size. More preferably, the span is less than 0.8.
A particularly desirable and surprising aspect of the present invention is that the toner particles may be made to have an irregular surface texture that increases the surface area and thus substantially improves the triboelectric charging characteristics of the toner composition especially the charging speed. A fast triboelectric charging toner composition is particularly desirable when the toner composition is used in a mono-component development systems which are widely employed in desktop laser printers. Desired level of the irregular surface texture of the toner particles may be characterized by the surface roughness index greater than 1.2, the surface roughness index being defined as the ratio of surface areas of the irregular textured particles and smooth texture particles.
In general, it may be possible to achieve surface roughness indices of greater than 1.2 or so and up to as high as 5 or more and span values of the particle size distribution of less than 0.8 down to 0.5 or even 0.2.
Any suitable method may be employed to prepare the particulate resin composition. Some of the typical methods are; air jet milling, dispersion polymerization and, more preferably, and dispersion comminution. The dispersion comminution process is preferred because the particle produced with the process may have a micro-serrated surface texture and therefore a faster triboelectric charging behavior.
Any suitable dye may be used in the practice of the present invention so long as it can be bound to the particulate polymer resin. Preferred dyes include basic dyes, acid dyes, or reactive dyes. The weight ratio to dye to particulate polymer resin is generally from about 1:100 to about 10:100 or from about 1 to about 10 percent by weight.
Any organic solvent that does not dissolve the polymer resin may be used in the present invention. The immiscibility between the solvent and the resin may be assured by selecting the solvent (i.e., organic dispersion medium) with a solubility parameter different from that of the resin. The solubility parameter value of the organic solvent is different than the solubility parameter value of the particulate polymer resin by at least about 1. More preferably the solubility parameter of the organic solvent is different than the solubility parameter value of the particulate polymer resin by at least about 2. Particularly preferred are paraffinic solvents.
A surfactant is preferably included in the inventive process. Most preferred are non-ionic surfactants as detailed further herein. Especially useful non-ionic surfactants include copolymers of vinylpyrrolidinone, copolymers containing ethylene oxide moieties or propylene oxide moieties.
The surfactant may be present in an amount of from about 5 to about 200 percent by weight of the amount of organic solvent present in the organic medium, whereas from about 10 to about 50 percent is more typical with from about 20 to about 40 weight percent of surfactant being preferred.
Any solvent that has some solubility of the functionalized dye may be used as the dye-mediating co-solvent. However, it is preferable for the dye-mediating co-solvent or dyeing assistive to be soluble in the surfactant for expeditious dyeing and has a limited solubility of less than about 5 percent by weight in the polymer resin to insure that there is no agglomeration of the particles during dyeing. Furthermore, a dyeing assistive with a low boiling temperature below about 100xc2x0 C. is preferred so that the dyeing operation may be carried out at a low temperature and thereby the micro-serrated surface texture may be retained after the dyeing operation. Examples of the preferred dyeing assistives are; ethyl alcohol; propyl alcohol; acetone; tetrahydrofuran; methyl ethyl ketone; butanone; water and a combination thereof. Any suitable co-solvent may be employed provided it has some miscibility with both the dye and resin of the particles.
The dye-mediating co-solvent may be present in an amount of from about 1 to about 30 percent by weight of the amount of organic solvent present in the organic medium, whereas from about 5 to about 20 percent is preferred.
It is likewise preferred to operate the inventive process at relatively high solids content wherein the polymer resin is present in an amount of from about 10 to about 70 volume percent of the combined volume of resin and organic medium during dying. From about 20 to about 40 volume percent resin is perhaps more typical in some embodiments.
The elevated temperature at which the process of the invention is carried out is generally lower than 30xc2x0 C. more than the glass transition temperature of the resin being dyed. For example, a resin having a glass transition temperature of 80xc2x0 C. is dyed at a temperature lower than about 110xc2x0 C. During the dyeing process the organic medium is maintained at an elevated temperature which is typically close to the glass transition temperature of the particulate polymer resin so that the dye and the charge control agent can readily penetrate the resin without substantially altering the surface texture of the resin particles. Particularly preferred in some embodiments is an elevated temperature the same as glass transition temperature of the polymer resin. Typically the polymer is dyed for at least five minutes and in many embodiments between about 5 and about 60 minutes.
A charge control agent is preferably added during the step of dyeing the particulate resin so as to simplify processing. Charge control agents are discussed in more detail hereinafter.
There is provided in still another aspect of the present invention a dispersion dyed color toner for developing latent electrostatic images. The inventive toner is prepared by a process including dispersing a particulate polymer resin provided with functional sites suitable for interacting with a functionalized dye in a liquid organic medium comprising an organic solvent, a surfactant and a dyeing assistive, the polymer being substantially insoluble in the organic solvent; providing the functionalized dye to the organic medium, wherein the functionalized dye has functional sites adapted for interacting with the functional sites on the particulate polymer resin; maintaining the organic medium, containing the particulate polymer resin and the dye at an elevated temperature for a time sufficient to dye the resin; and separating the organic medium from the particulate polymer resin. The functionalized dye is thus applied to the resin particles and the particle size of the particulate polymer resin is substantially unchanged during the process of preparing the toner.
Preferably, the toner particles are substantially spherical, have the volume average diameter in the range of from about 2 microns to about 10 microns, have a narrow size distribution with the span value less than 1.0 and further have micro-serrated surface texture characterized by the surface roughness index value larger than 1.2.
In most embodiments the color toner also includes a charge control agent present in an amount from about 0.1 weight percent to about 10 percent by weight of the toner. The toner may optionally include a flow improvement agent such as fumed silica.
There may be prepared developer compositions comprising the dispersion dyed color toner of the present invention. The developer composition includes the toner and carrier particles selected from the group consisting of ferrite particles, steel powder, iron powder and the like having a surface active agent coated therein. Examples of suitable carrier composition are described in U.S. Pat. No. 5,693,444, the disclosure of which is incorporated herein by reference.